This invention relates to the coding of video signals, and more particularly to a coding scheme that is capable of simultaneously transmitting both low quality coded video signals using existing coding standards, and high quality coded video signals.
The CCITT has recently recommended a coding scheme for video-phone and video-conferencing services ("Video Codec for Audiovisual Service at p.times.64 kbits/s" CCITT recommendation H.261, CCITT/COMXV/RAPP/R03TE4.TXS). This standardized codec is designed to operate at very low speeds (i.e., at multiple rates of 64.times.n(n=1-32) kb/s for video-phone and video-conferencing applications). For video-conferencing the spatial and temporal resolution of the input is set at 352 (pels) [reduced from original 360 pels so as to be evenly divisible into blocks 16 pels wide].times.288 (lines) and 29.97 frames/sec, respectively (Common Intermediate Format, CIF). For video-phone, where the transmission rate is lower (i.e., 64 kb/s), the spatial resolution is reduced to 1/4 CIF (i.e., 176.times.144). Consequently, at these speeds and resolutions, the codec may be unable to produce video with the quality that can support all the needs of the business community. The proposed standard, however, is expected to provide worldwide availability of the CCITT codec at a reasonably low cost.
With the expected wide deployment of optical facilities in the near future, and the availability of broadband systems such as Broadband ISDN (BISDN), a wider range of higher quality digital video services will likely be in demand, where quality is generally meant to mean spatial resolution (i.e., the number of pels per scan line.times.the number of scan lines per video frame). It is reasonable, therefore, to expect a wide range of quality video services, such as Extended Quality Television (EQTV), High Definition Television (HDTV), and future super HDTV, to be available together with the lower-quality video services such as the video-phone and video-teleconferencing services. To maximize the integration of all these various quality video services, a single coding system which can provide an unlimited range of video services is desirable. Such a coding system would enable users of different qualities to communicate with each other. For example, a subscriber to only a lower quality grade of video service should be capable of decoding and reconstructing a digitally transmitted higher quality video signal, albeit at the lower quality service level to which he subscribes. Similarly, a higher quality service subscriber should be capable of decoding and reconstructing a digitally transmitted lower quality video signal although, of course, its subjective quality will be no better than its transmitted quality.
In addition to enabling different quality grade video service subscribers to communicate with each other, a system architecture that utilizes basic hardware building blocks for all levels of service would permit the upward migration of a lower quality grade video service subscriber to a higher quality grade service subscriber without the costly expense of replacing his already installed lower quality grade video service hardware.
In a co-pending patent application of the inventor herein, Ser. No. 673,958, filed Mar. 22, 1991, a multilayer coding algorithm for the transmission of high quality video with full compatibility to the CCITT H.261 standard coder/decoder for video-phone is disclosed. The coding algorithm of the present invention has improved efficiency in terms of compression. Furthermore, the efficiency of the algorithm of the present invention remains substantially unaffected by the constraint that it remain compatible with existing lower video quality standards.